Vulnerability in io.modelcontextprotocol.sdk:mcp-core (CVE-2026-34237)
Summary
Vulnerability in io.modelcontextprotocol.sdk:mcp-core (CVE-2026-34237). Risk of unauthorized operations or information disclosure. Mitigation: upgrade to 0.18.3 or later.
Original description
The description as published by the source.
Summary
Hardcoded Wildcard CORS (Access-Control-Allow-Origin: * )
- https://github.com/modelcontextprotocol/java-sdk/blob/main/mcp-core/src/main/java/io/modelcontextprotocol/server/transport/HttpServletSseServerTransportProvider.java#L289
- https://github.com/modelcontextprotocol/java-sdk/blob/main/mcp-core/src/main/java/io/modelcontextprotocol/server/transport/HttpServletStreamableServerTransportProvider.java#L525
Attack Scenario
An attacker-controlled web page instructs the victim's browser to open GET https://internal-mcp-server/sse. Because Access-Control-Allow-Origin: * allows cross-origin SSE reads, the attacker's page receives the endpoint event — which contains the session ID. The attacker can then POST to that endpoint from their page using the victim's browser as a relay.
Comparison with python-sdk
No Access-Control-Allow-Origin header is emitted by either Python transport. The browser's default same-origin policy remains in full effect.
https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/server/sse.py
https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/server/streamable_http.py
Recommendation
In the SDK, the transport layer should not own CORS policy. Server implementors who need cross-origin access can add a CORS filter at the servlet filter or Spring Security layer.
Reference
Source: GitHub Security Advisories (Global) NVD GitHub Advisory
Auto-generated summary
Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.
A vulnerability tracked as CVE-2026-34237 has been found in io.modelcontextprotocol.sdk:mcp-core.
Risk of unauthorized operations or information disclosure. CVSS score: 6.1/10.
What to do: upgrade io.modelcontextprotocol.sdk:mcp-core to 0.18.3 or later.
If unsure, ask your IT team or search "io.modelcontextprotocol.sdk:mcp-core CVE-2026-34237" on the vendor's site.
CVE-2026-34237 (io.modelcontextprotocol.sdk:mcp-core) — CWE-942 / CVSS v3 6.1
Attack vector: remote (network-reachable) / unauthenticated
Patched: 0.18.3 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.
A vulnerability (CWE-942) exists in io.modelcontextprotocol.sdk:mcp-core.
Check the vendor advisory for the affected versions of io.modelcontextprotocol.sdk:mcp-core.
Severity: Medium (CVSS 6.1/10). Risk of unauthorized operations or information disclosure.
Update to 0.18.3.
Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.
Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Audit SBOM/dependencies to find affected services.
Response actions
Concrete steps and command examples for SOC/SRE teams to execute in order
-
1Identify exposure identify
Audit SBOM/dependencies for affected components.資産台帳・SBOM・依存マニフェストで io.modelcontextprotocol.sdk:mcp-core を使っているサービスとバージョンを洗い出す。
-
2Apply patch patch
Upgrade io.modelcontextprotocol.sdk:mcp-core to 0.18.3ステージング環境で 0.18.3 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。
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3Post-deployment verification verify
Confirm patched version is live in productionパッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。
Affected packages
References
- advisory nvd.nist.gov
- package github.com
- patch security-advisories@github.com
- patch security-advisories@github.com
- patch security-advisories@github.com
- web cheatsheetseries.owasp.org
- web github.com